Rail road cars in North America commonly employ double axle swiveling trucks known as “three piece trucks” to permit them to roll along a set of rails. The three piece terminology refers to a truck bolster and pair of first and second sideframes. In a three piece truck, the truck bolster extends cross-wise relative to the sideframes, with the ends of the truck bolster protruding through the sideframe windows. Forces are transmitted between the truck bolster and the sideframes by spring groups mounted in spring seats in the sideframes. The sideframes carry forces to the sideframe pedestals. The pedestals seat on bearing adapters, whence forces are carried in turn into the bearings, the axles, the wheels, and finally into the tracks. The 1980 Car & Locomotive Cyclopedia states at page 669 that the three piece truck offers “interchangeability, structural reliability and low first cost but does so at the price of mediocre ride quality and high cost in terms of car and track maintenance.”
Ride quality can be judged on a number of different criteria. There is longitudinal ride quality, where, often, the limiting condition is the maximum expected longitudinal acceleration experienced during humping or flat switching, or slack run-in and run-out. There is vertical ride quality, for which vertical force transmission through the suspension is the key determinant. There is lateral ride quality, which relates to the lateral response of the suspension. There are also other phenomena to be considered, such as truck hunting, the ability of the truck to self steer, and, whatever the input perturbation may be, the ability of the truck to damp out undesirable motion. These phenomena tend to be inter-related, and the optimization of a suspension to deal with one phenomenon may yield a system that may not necessarily provide optimal performance in dealing with other phenomena.
In terms of improving truck performance, it may be advantageous to be able to obtain a relatively soft dynamic response to lateral and vertical perturbations, to obtain a measure of self steering, and yet to maintain resistance to lozenging (or parallelogramming). Lozenging, or parallelogramming, is non-square deformation of the truck bolster relative to the side frames of the truck as seen from above. Self steering may tend to be desirable since it may reduce drag and may tend to reduce wear to both the wheels and the track, and may give a smoother overall ride.
Another issue which may arise may pertain to peak loading in the rollers of the bearings. It is thought that the life of bearing components may be strongly related to the maximum cyclic load. In some instances, the cyclic load may reach a maximum when the uppermost roller in a bearing race is at the top center position, with a steep drop off to either side of the topmost roller. It may be desirable to spread this loading in an effort to moderate the peak loading as the rollers pass through the top center position.